You can call me Whatever you like

The docs spend many words to explain in great detail what a Whatever is and how to use it from the caller perspective. There are quite a few ways to support Whatever as a callee as I shall explain.

Whatever can be used to express “all of the things”. In that case we ask for the type object that is Whatever.

sub gimmi(Whatever) {};
gimmi(*);

Any expression that contains a Whatever * will be turned into a thunk. The latter happens to be a block without a local scope (kind of, it can be turned into a block when captured). We can ask specifically for a WhateverCode to accept Whatever-expressions.

sub compute-all-the-things(WhateverCode $c) { $c(42) }
say compute-all-the-things(*-1);
say (try say compute-all-the-things({$_ - 1})) // 'failed';
# OUTPUT: «41␤failed␤»

We could also ask for a Block or a Method as both come preloaded with one parameter. If we need a WhateverCode with more then one argument we have to be precise because the compiler can’t match a Callable sub-signature with a WhateverCode.

sub picky(WhateverCode $c where .arity == 2 || fail("two stars in that expession please") ) {
    $c.(1, 2)
}
say picky(*-*);
# OUTPUT: «-1␤»
say (try picky(*-1)) // $!;
# OUTPUT: «two stars in that expession please␤  in sub picky at …»

The same works with a Callable constraint, leaving the programmer more freedom what to supply.

sub picky(&c where .arity == 2) { c(1, 2) }

There are quite a few things a WhateverCode can’t do.

sub faily(WhateverCode $c) { $c.(1) }
say (try faily( return * )) // $!.^name;
# OUTPUT: «X::ControlFlow::Return␤»

The compiler can take advantage of that and provide compile time errors or get things done a little bit qicker. So trading the flexibility of Callable for a stricter WhateverCode constraint may make sense.